Shallow trench isolation and plug technology have been used in the semiconductor industry to reduce circuit topography and better isolate adjacent semiconductor devices. In the early stages of trench isolation technology, silicon dioxide (SiO.sub.2) was used to form plug regions for trenches and various topographical structures.
SiO.sub.2 trench plugs improved isolation and topography, but also created undesirable mechanical stress and strain in integrated circuits. SiO.sub.2, due to the fact that it was nonconformal, could not fill narrow trenches or plugs. When SiO.sub.2 is used to form plugs in narrow trenches or trenches with a high aspect ratio, a known and undesirable phenomenon called crevice formation occurs. The compatibility of materials with narrow trenches or plugs is important because narrow trenches and plugs are becoming widely used as integrated circuit cell sizes decrease and photolithographic minimum geometry sizes shrink. Another disadvantage of the use of SiO.sub.2 for trench plugs is that SiO.sub.2 formations and layers cannot be reflowed and therefore are less planar than desired.
Due to the fact that SiO.sub.2 could not be reflowed, boro-phosphate-silicate-glass (BPSG) was proposed as a plug fill material. BPSG is capable of being reflowed in a desirable planar manner for use as a plug material. Improved planarization helped to reduce the topography of plug etch processing. Although BPSG was capable of being reflowed, BPSG is a doped material. The dopant atoms in BPSG, phosphorus and boron, are mobile and can outwardly diffuse at higher temperatures. Therefore, when subsequent process steps occur, boron and phosphorus penetrate adjacent areas and are the cause of various problems such as threshold voltage shifting, reduced isolation, and oxide contamination. In addition, BPSG reduced trench mechanical stress and strain when compared to SiO.sub.2, but the levels of BPSG stress and strain are still above desired trench stress levels.